The invention is based on an appliance for discharge lamps as claimed in the precharacterizing clause of claim 1. In particular, the invention relates to the starting of these lamps.
Appliances for discharge lamps, referred to as lamps for short in the following text, have, according to the prior art, an AC voltage generator which is connected via a load circuit to one or more lamps. The AC voltage generator is normally in the form of a half-bridge or full-bridge inverter with electronic switches, so that the AC voltage generator emits a square-wave source voltage Uq. The load circuit is essentially a reactance network, whose object, inter alia, is to transform the source impedance of the AC voltage generator to a value required for operation of lamps.
Particularly for operation of low-pressure lamps, the load circuit also carries out the function of starting these lamps. To this end, the load circuit is designed such that it has a resonance point. This means that the load circuit is able to produce a high voltage, which is suitable for starting a lamp, at the output when excited at its resonant frequency fres. In the simplest case, the load circuit comprises an inductance L and a capacitance C connected in series. This circuit has a resonant frequency fres at xc2xdxcfx80{square root over (LC)}. A lamp is connected in parallel with the capacitance C. If the AC voltage generator feeds a square-wave source voltage Uq at a fundamental frequency corresponding to the resonant frequency fres into the load circuit, then this results in a voltage increase across the capacitance C, which leads to the lamp being started. Accordingly, the prior art strives to satisfy the following condition Tperxc2x7fres =1 for the period duration Tper of the source voltage Uq. The desired value for the duty ratio of the source voltage Uq is 0.5. The duty ratio is the ratio of the pulse duration Tpulse to the pulse pause in the square-wave source voltage Uq.
One problem when starting lamps in the way described above is that the fundamental frequency of the source voltage Uq which is emitted from the AC voltage generator must be set accurately, since the resonance point of the load circuit in general has a narrow bandwidth. The resonant frequency fres must be produced with an accuracy of better than 1% since, otherwise, the voltage across the capacitance C will not be increased sufficiently for the lamp to be started reliably. If the resonance point of the load circuit has a high Q-factor, then it is also possible for a problem to occur due to the resonant frequency fres being produced too accurately. In this case, current and voltage amplitudes which lead to destruction of components can occur in the appliance.
So-called self-excited generators are known for use as the AC voltage generator. In these generators, the drive signal for the electronic switches in the AC voltage generator is obtained from the load current. This results in a self-regulating effect, which places the frequency of the source voltage Uq, which is emitted by the AC voltage generator, in the vicinity of the resonant frequency fres. However, such self-excited generators can be used to only a limited extent to control lamp operation, for which reason so-called externally excited generators are used increasingly more frequently. In externally-excited generators, an independent oscillator produces the drive signal for the electronic switches in the AC voltage generator. Independent means that, in contrast to self-excited generators, an oscillation can be produced which is independent of variables such as the load current or load voltage.
In the prior art, a number of solutions have been proposed with the aim of setting the frequency of an independent oscillator, as mentioned above, such that the lamp is started reliably.
The document EP 0 351 012 (Wong) proposes that the frequency of the independent oscillator be set to a value which is above the resonant frequency fres, and that it is then reduced continuously until it reaches the resonant frequency fres. However, this results in the problem that, firstly, the change in the frequency of the independent oscillator may not be too fast to allow the resonance to build up in the load circuit while, on the other hand, the lamp should be started as quickly as possible in order that preheated filaments in a lamp do not cool down again before starting.
The document EP 0 831 678 (Nerone) proposes that a closed control loop be used to control the frequency of the independent oscillator such that the desired starting voltage is produced across the lamp. Since, as already mentioned, the resonance point of the load circuit has a narrow bandwidth, the proposed control system is highly complex.
A further problem is that the independent oscillator ever more frequently makes use of digital technology. This may be done by using a microcontroller. Digital technology means that the independent oscillator can no longer produce any desired frequency. Only discrete frequencies can be produced, and these are predetermined by a fixed interval. In order to produce the resonant frequency fres with sufficient accuracy, a high level of complexity must be accepted in order that the predetermined interval allows sufficiently good resolution for the frequencies which can be produced.
One object of the present invention is to provide an appliance as claimed in the precharacterizing clause of claim 1, which allows reliable starting of discharge lamps, with little complexity.
This object is achieved for an appliance having the features of the precharacterizing clause of claim 1 by the features in the characterizing part of claim 1. Particularly advantageous refinements can be found in the dependent claims.
As stated above, an AC voltage generator for starting lamps produces a square-wave source voltage Uq whose fundamental frequency is close to the resonant frequency fres. According to the prior art, this source voltage Uq, once it has been set, is applied continuously throughout the entire starting process. When this source voltage Uq is switched on, a transient oscillation process is evident, which forms a transient overvoltage across the lamp. Although the amplitude of this overvoltage is sufficient to start the lamp, it is too short to actually start it. The present invention makes use of the transient overvoltage by triggering transient oscillation processes in a periodic sequence according to the invention. To this, the AC voltage generator produces a square-wave source voltage Uq whose period duration Tper attempts, according to the invention, to satisfy the following condition: Tperxc2x7fres=n where n is a natural number greater than 1. In order to achieve a sufficiently high voltage to start the lamp, the following condition: 0.3xe2x89xa6Tpulsexc2x7fresxe2x89xa61 must be satisfied according to the invention, for the pulse duration Tpulse of the source voltage Uq.
The profile of the source voltage Uq according to the invention has the advantage that this reduces the accuracy requirement for the independent oscillator by a factor of at least two. This means that the condition Tperxc2x7fres=n according to the invention mentioned above need be satisfied only approximately. In this context, approximately means that the condition is satisfied with an accuracy of 3%. Even less accuracy is sufficient for higher values of n.
The profile of the source voltage Uq according to the invention has the further advantage that the capability to select Tper and Tpulse makes it possible to adjust the amount of energy which the AC voltage generator feeds into the load circuit. It is thus possible to prevent components from being damaged if the load circuit is excited at precisely its resonant frequency.
For high values of n, the described transient oscillation process is triggered only rarely. This means that the lamp may no longer be started reliably. It has been found that a value of n=3 ensures that the lamp starts reliably.
The profile of the source voltage Uq according to the invention is preferably fed into a load circuit which contains an inductance and a capacitance connected in series. This results in the minimum complexity to achieve the desired resonance.
The profile of the source voltage Uq according to the invention is preferably produced by an independent oscillator since, as explained above, this makes it simple to influence the profile of the source voltage Uq.
It is particularly advantageous for the independent oscillator to use digital technology. This allows the profile of the source voltage Uq to be modified just by changing register contents. This can be done just by a software change, if a microcontroller is used.
The complexity to provide the AC voltage generator according to the invention is then very low, provided the electronic switches contained in it can be driven directly from the independent oscillator.